To improve the cold start behavior of a diesel engine, so-called glow plugs are used in diesel engines. These glow plugs are screwed tightly into a mounting hole of a cylinder head of the diesel engine. The glow plugs pass through the cylinder head into the respective corresponding combustion chamber, so that the latter can be preheated by the glow plug when needed before the diesel engine is started. In the area of the combustion chamber-side end the glow plugs have a heating element, which protrudes at least minimally into the combustion chamber of the corresponding cylinder of the diesel engine.
This heating element is accommodated by a mounting tube, which is provided with a mounting screw in its end area located opposite the heating element. The mounting hole of the cylinder head correspondingly also has a corresponding internal thread in its axially outer end area, and the mounting tube with its mounting thread can be screwed into this internal thread. The mounting hole has a radially tapered section, by which a sealing seat is formed, toward the combustion chamber. The mounting tube of the glow plug is adapted to this offset design of the mounting hole and has a circular sealing surface, with which the mounting tube comes tightly into contact with the sealing seat of the mounting hole when it is screwed in, in a corresponding axial arrangement.
To screw in the glow plug, the mounting tube is provided with a hexagon insert bit made in one piece axially outside the mounting thread, so that the glow plug can be screwed into the mounting hole or the internal thread of the mounting hole by means of a conventional socket wrench.
After a longer operating time of the diesel engine, it is frequently necessary to replace defective glow plugs. It can frequently be observed in this connection that the mounting tube with its mounting thread is seated extremely tightly in the internal thread of the mounting hole, so that the hexagon insert bit is shorn off and the mounting tube with its mounting thread thus remains in the internal thread during the attempt to screw the mounting tube and thus the entire glow plug out of the mounting hole via the hexagon insert bit.
A contact pin, which has a contact element on the outside, is inserted into the mounting tube for the electric connection. After the hexagon insert bit has been shorn off, it is now necessary to remove first this contact pin together with the contact element in order to make it possible to remove the hexagon insert bit from the rest of the mounting tube. A conventional socket nut, which is attached to the contact element by pressing or hammering, is normally used for this purpose, and this contact element now assumes the contour of the socket nut. The socket nut may be designed as an inner Torx or hexagon socket. By subsequently rotating the socket nut by means of a suitable tool, the contact element is now shorn off together with the contact pin, so that these can be removed from the mounting tube. The mounting tube is thus accessible now from the outside.
To remove the mounting tube and thus the rest of the glow plug with the heating element thereof from the mounting hole, the mounting tube is now drilled out with a so-called core hole drill as a next step, as a result of which the mounting thread of the mounting tube is removed, possibly without damaging the internal thread of the mounting hole. An internal thread can subsequently be tapped in the rest of the mounting tube by means of a conventional screw tap and pulled out by means of a draw spindle or the like. The internal thread of the cylinder head is subsequently to be finished by means of a corresponding screw tap in order to make it possible to insert a new glow plug.
Furthermore, provisions may be made for removing combustion residues or other contaminants, for example, oxidation products, from the mounting hole by means of a reamer before the new glow plug is inserted, so that the mounting hole will again assume, at least approximately, its original new state.
Both when the mounting thread is drilled out and when the internal thread of the cylinder head is retapped, it is necessary to align the corresponding tool coaxially with the central longitudinal axis of the mounting hole with the highest precision in order to ensure that a new glow plug will be screwed in absolutely satisfactorily centrally in relation to the hole in the cylinder head. This can be achieved with the conventional tools or tool systems with difficulty only.